1. Field of the Invention
The present invention relates to an alkaline battery sealing gasket and a sealed alkaline battery, and particularly to a technology effective when applied to an alkaline dry battery having an anti-burst function.
2. Description of the Related Art
Alkaline dry batteries commonly called by their type name such as AM contain an electricity generating element in their metal battery can that doubles as a positive electrode terminal and the opening of the battery can being sealed by their negative electrode terminal and resin sealing gasket. The sealing gasket has a peripheral packing portion and an isolation wall portion. By the peripheral packing portion being sandwiched in a pressed state between the opening end of the battery can and the negative electrode terminal, the opening of the battery can is sealed in an airtight manner. The isolation wall portion isolates a space on the back side of the negative electrode terminal from a space containing the electricity generating element. By forming a thinner portion into groove-like in the isolation wall and breaking down the thinner portion earlier with a gas pressure increase in the containing space, a safety valve function to prevent increases in gas pressure from causing a burst of the battery can be achieved. By this means, an anti-burst, sealed alkaline battery can be made. See, for example, Japanese Patent Application Laid-Open Publications No. 07-105925 and No. 2002-251987.
In terms of maintaining the high sealability of the alkaline dry battery for a long time, it is considered preferable that the sealing gasket is excellent in both flexibility and impact resistance as well as alkaline resistance. Hence, an olefin-based resin is adopted as the material of the gasket. In particular, a polypropylene (hereinafter, PP) resin is considered a useful material excellent in moldability as well, but is somewhat deficient in terms of impact resistance. In order to solve this problem, a PP resin with its degree of crystallization increased and a PP resin with a rubber-based resin added have been used.
The impact resistance of resins used for the sealing gasket is evaluated in terms of an Izod impact value (Izod impact test value) according to a JIS-K7110-1984 measurement method. In general, firm materials are brittle, and soft materials have viscosity. For resins (plastics), the Izod impact value is often used as an index to evaluate the tenacity of these materials.
The Izod impact value indicates the magnitude of impact energy absorbed by a material (resin) under test, and is small for brittle materials and large for viscous materials.
In order to maintain the high sealability of the alkaline dry battery for a long time, a resin flexible and excellent in the capability of impact absorbing, that is, a flexible resin with a high Izod impact value is desirable. Hence, for conventional alkaline battery sealing gaskets, a PP resin having the Izod impact value increased to 80 or greater has been used as described in, for example, Japanese Patent Application Laid-Open Publication No. 63-166140.
However, it was revealed by the inventors that there is the following problem with the above conventional technology.
That is, with the sealing gasket having the safety valve function by a thinner portion, the safety valve function is activated with the thinner portion breaking open due to increases in gas pressure inside the battery. With a conventional sealing gasket having the safety valve function using a high impact-resistant resin (having an Izod impact value of 80 or greater), the activation property of the safety valve function in response to a rapid increase in gas pressure inside the battery is favored, but when the pressure inside the battery gradually increases, a creep phenomenon of the resin occurs, and thus the safety valve function may not be certainly activated. In other words, it was revealed that the problem occurs that the activation property of the safety valve function is poor thus increasing the risk of battery burst.
The creep in this case is a property characteristic of resins (plastics), and is a phenomenon that the deformation amount gradually increases as a constant force continues to be applied for a long time. In a metallic molded resin, molecules are oriented in the flow direction of the resin being injected into the metallic mold, and the creep phenomenon of the thinner portion appears to a greater degree in the oriented direction. Hence, the thinner portion is desirably formed so as to break open along the oriented direction, and to do that, the thinner portion is desirably formed by a groove extending in the same direction as the oriented direction. Even in this case, however, in order to make sure that the thinner portion breaks open at a predetermined gas pressure, the resin needs to be made brittle, that is, its Izod impact value needs to be made low.
However, if a brittle resin having a low Izod impact value is used, the original function of the sealing gasket, particularly the sealing function at the peripheral packing portion is reduced in effect. Thus, the contradiction arises that the high sealability of the battery cannot be maintained for a long time. While the sealing gasket can seals the battery can airtight by its peripheral packing portion being sandwiched in a pressed state between the opening end of the battery can and the negative electrode terminal, the quality of sealing function is greatly depending upon the flexibility of the resin. It has been considered that if a quality sealing function is required for the sealing gasket, the resin that is the material for it must have flexibility and be a high impact-resistant resin (PP) having at least an Izod impact value of 80 to 100.
Meanwhile, when the safety valve is of interest, it is found that the high impact-resistance is not necessarily useful to the activation property of the safety valve function, but rather hurts its activation property. In other words, with alkaline battery sealing gaskets, there is a contradiction between the sealing function to maintain the high sealability of the battery for a long time and the safety valve function to certainly prevent battery burst. However, this has not been considered to be a problem conventionally. This is supposedly because when testing the activation of the safety valve function, gas pressure inside the battery was rapidly increased. In the case of the alkaline dry battery, a test of the safety valve function can be performed where by forcing a charge current to flow the battery, reactant gas is generated inside the battery. In order to perform this test efficiently, the gas generation speed needs to be increased by forcing a large charge current to flow. As such, by rapidly increasing gas pressure, the test can be performed efficiently in a short time, but by this means, the activation property in which the gas pressure gradually increases cannot be examined.
The inventors have acquired knowledge that the activation of the safety valve function is greatly different depending on how the gas pressure increases and even if the gas pressure rapidly increases with a quality activation property, the activation property becomes worse when the gas pressure increases gradually. That is, it has become apparent that with the above conventional alkaline battery sealing gaskets, there is the problem that the sealing function to maintain the high sealability of the battery for a long time and the activation property of the safety valve function for when gas pressure inside the battery gradually increases are not compatible.